In this screenshot, I have several different mez resist bonuses which are listed as "-X.XX% duration". This seems to imply that each of these effects subtracts that listed percentage from the duration of effects. However, this is apparently not the case; status effect resistance, I am told, follows the same rules as recharge reduction. 100% amounts to a decrease by half, because it's 1/(1+Resist%), rather than 1-(Resist%). This can even be seen in the actual duration listed as the header for each type of resistance. 57.55% duration implies that my total resistance is -42.45% duration, which is much lower than the sum of all of the values listed beneath; they are thus not actual -% durations. Therefore, framing these buffs as "-X.XX% duration" is incredibly misleading. One would expect, for example, that if I had one -9.09% duration buff, my actual duration would be 90.91%, and that two -9.09% duration buffs would result in a duration of 81.82%, and so on.
So, I think the display of these buffs should be adjusted. For example, calculate the actual percentage each reduces the duration by and display that value. At a minimum, the labeling of the displayed values should be adjusted to something like "9.09% resistance" or, better yet, just "10% resistance" since that's the actual buff amount from the set bonus. Calculating the reduction in duration assuming you only had that buff is ind of pointless; those values have no correlation with the actual effect they all have together on your mez duration. Ideally, I think that the best solution would be to do both, similar to how HP and End regen/recovery is displayed to show the base value as well as the actual value considering whatever your maximum HP and End pools can be. For example, all of the Ultimate ___ and Mez Resistance buffs would be "10% resistance (-5.76% duration)" since each of them contributes 5.76 percentage points of the 42.45% duration reduction.